The granular monohydrated sodium perborate (NaBO.sub.2.H.sub.2 O) is generally obtained by dehydration of the so-called tetrahydrated salt (NaBO.sub.2.H.sub.2 O.sub.2.3H.sub.2 O) in the granular state and is mainly used as a bleaching agent for detergent formulations.
The monohydrated salt is essentially showing three advantages in comparison with the tetrahydrated salt: a higher content of available oxygen, a higher heat stability and a higher dissolution rate into water. However, it is known that the monohydrated salt is remarkably more brittle than the tetrahydrated one. One of the main reasons which have hitherto limited its exploitation is residing in the fact that, as a consequence of the manufacture of the product, the brittleness involves a variation of the distribution of the particle size and an enriching of the powder with the finest fractions. The main users of perborate do not want to have a product showing a high content of fine powder, in that it causes serious hygienic and ecological problems during the mixing of the components and can favour the caking of the detergent when it is manufactured. In order to avoid that the monohydrated salt, owing to its brittleness, be crushed by the pneumatic conveyors (everywhere employed by manufacturers and users) it is preferred to transport the monohydrated salt in bags rather than in bulk and all this involves higher burdens, as to either the packaging requirements or the reduced storage capacity. Hence the necessity of having a moohydrated salt endowed with good abrasion resistance, which can be conveyed in bulk without producing any fine powder. In the past two main ways were undertaken:
(a) control of the dehydration conditions of the tetrahydrated perborate and in particular control of temperature and moisture of the drying air (see for instance European Patents 155,894 and 194,952; U.S. Pat. No. 3,623,836, German Patent publications 2,444,780 and 2,258,319 and French Patent 1,081,421 (the content of which patents is an integrative part of the present application) or: PA0 (b) addition of specific additives to the monohydrated perborate (see for instance German Patent 2,203,285).
However, in both cases, it was impossible to obtain a product to be conveyed in bulk and to wholly satisfy the expectations of the users.
Another way for solving the problem was indicated in European Patent 202,519; according to this patent, in the name of the Applicant, the use of additives (in this particular case sodium meta-silicate) is not performed on the finished monohydrated salt but on the tetrahydrated salt before its drying. By this way, the melting point of the tetrahydrated salt (otherwise 63.degree. C.) is lowered down so that (during the starting step of dehydration on the particle surface) an initial melting takes place, by quick withdrawal of the molecules of crystallization water, which withdrawal gives the finished product the desired stiffness. By this way, it is possible to reach a decisive improvement of the abrasion resistance of the monohydrated perborate, by a simple way and independently of the drying method. However, the addition of meta-silicate does not allow to obtain a monohydrated perborate showing a particularly high content of available oxygen, because meta-silicate generally retains very firmly the crystallization water, which is not completely removed under the usual dehydration conditions of the tetrahydrated perborate. It had been tried since a long time to find new additives able to decrease the melting point of the tetrahydrated perborate (thus granting a high abrasion resistance of the corresponding monohydrate) and which do not contain any crystallization water at the end of the dehydration, thus assuring a sufficiently high content of available oxygen in the final product. The Applicant succeeded now in performing a process for obtaining granular monohydrated perborate endowed with a good abrasion resistance, which can be also transported in bulk and which exhibits a high content of available oxygen.